1. Field of the Invention
The present invention relates to an electrostatic recording apparatus for recording an image on a recording medium by using an output signal delivered out of a computer or an electrographic.
2. Description of the Related Art
Known as an apparatus, wherein an image carrying output signal from a computer or an electrographic is used to form an electrostatic latent image at a high rate on a recording medium having a charge holding layer and an electrically conductive layer and the electrostatic latent image is developed with toner into a visible image, are electrostatic recording apparatuses with multi-electrode recording head as disclosed in, for example, JP-A-53-20929, JP-A-61-10466 and U.S. Pat. No. 3,653,065. A recording head of the electrostatic recording apparatus disclosed in JP-A-61-10466 has a great number of electrically conductive elongated recording electrodes which are formed in parallel on each of opposite surfaces of a substrate made of an insulating material such as glass or epoxy resin. As will be seen from a section of the recording head shown in FIG. 8, recording electrodes 2 are formed on opposite surfaces 1a and 1b of a substrate 1 in such a manner that each electrode 2 on the surface 1a confronts a gap between adjacent electrodes 2 on the surface 1b. In recording operation, while a recording medium (not shown) making contact with ends of the recording electrodes is moved relative to the recording head substantially orthogonally to the longitudinal direction of the recording electrodes in a direction of arrow x, a series of binary voltage signals representative of an image are applied to the individual recording electrodes to form on the charge holding surface of the recording medium an electrostatic latent image representative of the image which is formed of many dots each having a shape similar to a sectional shape of each recording electrode. In order to obtain a square dot shape, the recording electrode 2 is so formed as to have a substantially square cross-section. For formation of the recording electrodes 2 as above, a thin film of copper is first deposited in a layer on the substrate 1 through, for example by, plating process and the copper thin film is covered with a resist film of a predetermined pattern by using printing technique. The thus prepared substrate structure is then etched so that copper is removed from portions not covered with the resist film and the remaining isolated portions of the copper thin film provide many elongated recording electrodes 2.
Recording electrodes 2 on one surface 1a of the substrate 1 are staggered with respect to recording electrodes 2 on the other surface in order to increase the density of dots to be printed. For example, when printing is first carried out along a line on a recording medium with the recording electrodes 2 formed on the one surface 1a and thereafter printing is again carried out along the same line with the recording electrodes 2 formed on the other surface 1b, the dot density which is twice that obtained by only the recording electrodes 2 on the one surface can be obtained.
When conducting recording by using the recording head as above, a recording paper acting as a recording medium (not shown) is so located as to slidably contact the recording electrodes 2 and an auxiliary electrode (not shown) disposed near the recording electrodes 2. After completion of printing along a line on the recording paper with the recording electrodes 2 formed on one surface 1a, the recording paper is moved in the x direction by a distance corresponding to the thickness of the substrate 1 and printing is carried out along the same line with the recording electrodes 2 formed on the other surface 1b. Upon completion of recording for the one line, the recording paper is again moved in the x direction and printing for the next line is carried out.
In the conventional electrostatic recording apparatus, recording for one line is carried out by sequentially applying binary signals representative of images to many recording electrodes through multiplexing technique. Thus, in order to print a drawing of, for example, AO size, 14,400 recording electrodes are used, of which 7,200 electrodes are formed on the surface 1a of the substrate 1 of FIG. 8 and the remaining 7,200 electrodes are formed on the other surface, 7,200 binary signals are first applied sequentially to the recording electrodes on the surface 1a and then 7,200 binary signals are similarly applied sequentially to the recording electrodes on the other surface 1b.
In order to make the cross-sectional shape of the recording electrode 2 nearly square, the electrode must have a thickness h of about 80 .mu.m to 120 .mu.m when the electrostatic recording apparatus is to provide a resolution or density of 400 dots/inch. The plating thickness which can be obtained through one cycle of copper plating process is however limited and therefore the thickness of the order mentioned above must be obtained through a plurality of cycles of plating process resulting in low productivity. Further, concurrently with removal of unnecessary copper coating between adjacent recording electrodes 2 through etching process, the side wall of the recording electrode 2 is also etched to form a scrape 3 in the side wall, making it very difficult to shape all the recording electrodes 2 on the substrate 1 into a square form. If any of the recording electrodes 2 on the substrate 1 have scrapes 3 as mentioned above, dots to be recorded are distorted in shape and quality of recording images are degraded disadvantageously.
Further, in order to print a drawing of, for example, AO size, the conventional electrostatic recording apparatus is so designed that about 14,400 recording electrodes 2 for one line are formed on the single substrate 1 and a series of binary signals representative of an image are sequentially applied in series to all of the recording electrodes 2. Therefore, the time required for scanning one line is about 14,400 times the time required for recording one dot. In addition, to meet the application of binary signals, signal lines must be provided individually for the 14,400 recording electrodes 2 with the result that stray capacitance between individual signal lines is increased and besides lengths of signal lines for different electrodes are greatly different from one another, thus adversely affecting quality of recorded images.